Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC

Novel proton-conducting polymer electrolyte membranes have been prepared from bacterial cellulose by incorporation of phosphoric acid (H₃PO₄/BC) and phytic acid (PA/BC). H₃PO₄ and PA were doped by immersing the BC membranes directly in the aqueous solution of H₃PO₄ and PA, respectively. Characterizations by FTIR, TG, TS and AC conductivity measurements were carried out on the membrane electrolytes consisting of different H₃PO₄ or PA doping level. The ionic conductivity showed a sensitive variation with the concentration of the acid in the doping solution through the changes in the contents of acid and water in the membranes. Maximum conductivities up to 0.08 S cm⁻¹ at 20 °C and 0.11 S cm⁻¹ at 80 °C were obtained for BC membranes doped from H₃PO₄ concentration of 6.0 mol L⁻¹ and, 0.05 S cm⁻¹ at 20 °C and 0.09 S cm ⁻¹ at 60 °C were obtained for BC membranes doped from PA concentration of 1.6 mol L⁻¹. These types of proton-conducting membranes share not only the good mechanical properties but also the thermal stability. The temperature dependences of the conductivity follows the Arrhenius relationship at a temperature range from 20 to 80 °C and, the apparent activation energies (E_a) for proton conduction were found to be 4.02 kJ mol⁻¹ for H₃PO₄/BC membrane and 11.29 kJ mol⁻¹ for PA/BC membrane, respectively. In particular, the membrane electrode assembly fabricated with H₃PO₄/BC and PA/BC membranes reached the initial power densities of 17.9 mW cm⁻² and 23.0 mW cm⁻², which are much higher than those reported in literature in a real H₂/O₂ fuel cell at 25 °C.